In a mobile unit such as an automobile, achievement of lightweight is required from the viewpoint of performance and/or environmental response. Therefore, two main approaches have been adopted so far to obtain structural materials that can meet such demands. One approach is to make an iron and steel material thin by raising strength thereof and another approach is to use an alloy having a small specific gravity instead of the iron and steel material.
However, when an iron and steel material is made higher in strength and smaller in thickness, problems arise such as degradation of ductility and fatigue strength and hydrogen embrittlement in association with the material being made stronger and thinner.
Moreover, in the case where the alloy of Al or Mg having a low specific weight is used instead of the iron and steel material, due to low strength and stiffness thereof, it is necessary to increase its plate thickness or to make its cross sectional shape complicated in order to obtain the strength and stiffness being equal to that of the iron and steel material. Consequently, another problem arises that it becomes impossible not only to achieve a lightweight effect that can be normally expected from differences in specific gravity but also to avoid the problems of degradation in press-formability, embrittlement caused by dissimilar materials joining, corrosion, and the like.
Therefore, the advent of an iron and steel material is strongly expected which can achieve structural lightweight thereof and can simultaneously have high strength and, further, can solve the problem of the degradation in ductility, toughness, formability, fatigue strength, corrosion resistance, resistance to hydrogen embrittlement, and the like coming with the lightweight and high strength.
However, since strength and ductility in particular are incompatible with each other, it is difficult for the conventional iron and steel materials to achieve the compatibility.
FIG. 1 is a graph showing a correlation between strength and ductility of conventional steels. In FIG. 1, strength is represented by tensile strength and ductility is represented by elongation. The steel shown as MART (Martensite) in FIG. 1 is martensitic steel having ultra-high strength but extremely low ductility. It can be understood from FIG. 1 that, in any of the steels, the higher the strength becomes, the lower the ductility becomes.
Conventionally, as one of means to improve properties of a material, the multilayering of the material by using different dissimilar materials each having required properties is being studied.
The material obtained by multilayering steels or other materials includes a clad plate, laminated plate, composite material, or the like.
The clad plate is a steel plate or metal plate obtained by performing surface plating of a highly corrosion-resistant steel or the like with the aim of imparting an anti-corrosion function. As a method of forming the clad plate, a method is proposed (for example, in Japanese Patent Application Laid-open No. Hei 5-5190) in which dissimilar steels are cold rolled and then annealed.
The laminated plate is a steel plate or metal plate obtained by sandwiching a resin or the like therebetween with the aim of imparting a damping function, thermal insulating function and the like. As a method of forming the laminated plate, a method is proposed (for example, in Japanese Patent Application Laid-open No. 2001-277271) in which a resin joined between two metal plates is melted and the two metal plates are pressure molded.
The composite material is a plate-shaped composite material produced by using a polymer, laminated foil such as a metal foil and carbon material with the aim of providing the material itself with high strength. As a method of forming the composite material made of dissimilar materials, a method is proposed (for example, in Japanese Patent Application Laid-open No. 2005-306039) in which composite parts made up of a prepreg sheet are compression molded. There is a report that the toughness and fatigue-resistant characteristic of the material itself are improved by multilayering the composite material.
In the conventional art, the clad plate is the steel plate or metal plate obtained by rolling dissimilar steels or metal plates and performing an annealing process or the like with the main aim of imparting a surface function or the like, not with the aim of making the structural material itself highly functional on the precondition that the materials are welded. Also, the laminated plate is the steel plate or metal plate obtained by melting the resin joined between two metal plates and performing pressure-molding with the aim of imparting a damping function, thermal insulating function, and the like, not with the aim of making the structural material itself highly functional on the precondition that the materials are welded.
The composite material is the plate-like material obtained by performing compression-molding on materials with the aim of making more stronger the plate-shaped multilayered material itself. There is a report that, by multilayering the composite material, toughness and fatigue strength of a brittle material such as a ceramic are improved. These findings were referenced to realize the present invention, however, studies of a balance between strength and ductility, toughness, formability, weldability, fatigue strength to which attention is being given in the present invention have not been reported.
The result is that, in the conventional multilayered materials, any structural material in which properties being incompatible with each other such as in the case of strength and ductility has not yet been achieved.
In view of solving the above problems, it is an object of the present invention to provide a multilayered steel of the steel/steel laminated type in which two properties of a steel being incompatible with each other such as in the case of high strength and high ductility can be made compatible with each other and which is excellent in strength, ductility, weldability, toughness, and fatigue strength and a method for producing the multilayered steel.